The communication of conformation states among protein isoforms is potentially an important mechanism for the transmission of physiological signals. The proposed investigation will examine the communication of ClpXP protease sensitivity among isoforms of bacteriophage Mu repressor, a protein that shuts down Mu transposition functions for the establishment and maintenance of lysogeny. The repressor serves to link expression of phage functions to specific physiological conditions. Trans-dominant, virulent forms of the Mu repressor (Vir) are altered in sequence at the C-terminus so that they are readily degraded by the chaperone-linked protease ClpXP of Escherichia coli. While unaltered forms of the repressor (Rep) are normally stable in vivo, Vir can promote rapid degradation of Rep by ClpXP. Biochemical analysis indicates that while Rep and Vir are both ClpXP substrates, the protease has a lower affinity for Rep. Vir can confer its properties as a high affinity substrate to many times its weight of Rep molecules. Current biochemical and genetic evidence suggests that Vir induces a conformational change in Rep to assume a high-affinity state and that this involves movement of its C-terminal tail which contains a determinant for ClpXP recognition. This may be part of a general mechanism for transducing physiological signals and promote repressor degradation to induce Mu transposition and host chromosomal rearrangement. Such a mechanism for propagating conformational changes among protein isoforms has precedence in mechanisms hypothesized for transmission of prion diseases (transmissible spongiform encelopathy) in mammals and for protein-based inheritance in yeast. The hypothesis that Vir communicates ClpXP hypersensitivity by transmitting conformational changes to Rep will be tested by accomplishing 3 specific aims: 1) Mutational analysis to define domains of repressor required for ClpXP recognition and for transmission of ClpXP sensitivity. 2) Biochemical characterization of mutationally altered Rep and Vir to determine their properties as ClpXP substrate and their ability to confer or acquire ClpXP hypersensitivity. 3) Probing physical differences between ClpXP-resistant and sensitive states of the Mu repressor. Conformational changes involved in transition between ClpXP- resistant and sensitive state of Rep, especially those that involve movement of its C-terminal tail, will be examined. The long-term objectives are to determine whether the propagation of protein conformational states can be a general mechanism for transducing signals that regulate key cellular processes.